Data and Computer Communications

Data and Computer Communications

Eighth Editionby William Stallings

Lecture slides by Lawrie Brown

Chapter 9 – Spread SpectrumChapter 9 – Spread Spectrum

Spread Spectrum important encoding method for wireless

communications analog & digital data with analog signal spreads data over wide bandwidth makes jamming and interception harder two approaches, both in use:

Frequency Hopping Direct Sequence

Spread Spectrum Input is fed into a channel encoder

Produces analog signal with narrow bandwidth Signal is further modulated using sequence of

digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number

generator Effect of modulation is to increase bandwidth of

signal to be transmitted

Spread Spectrum On receiving end, digit sequence is used to

demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data

Spread Spectrum Advantages

What can be gained from apparent waste of spectrum? Immunity from various kinds of noise and multipath

distortion Can be used for hiding and encrypting signals Several users can independently use the same higher

bandwidth with very little interference CDM/CDMA Mobile telephones

Pseudorandom Numbers generated by a deterministic

algorithm not actually random but if algorithm good, results pass

reasonable tests of randomness starting from an initial seed need to know algorithm and seed to

predict sequence hence only receiver can decode signal

Frequency Hoping Spread Spectrum (FHSS)

Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal (2k) Width of each channel corresponds to bandwidth of input signal

Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval (300 ms for LAN 802.11), a new

carrier frequency is selected

Frequency Hoping Spread Spectrum

Channel sequence dictated by spreading code Receiver, hopping between frequencies in

synchronization with transmitter, picks up message

Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only

at knocking out a few bits

Frequency Hoping Spread Spectrum

Frequency Hoping Spread Spectrum

Frequency Hoping Spread Spectrum

Frequency Hoping Spread Spectrum

Define the FSK input to the FHSS system as

Frequency Hoping Spread Spectrum Assume the duration of one hop is the same as the

duration of one bit ignore phase differences between the data signal and

the spreading signal, also called a chipping signal, c(t) .

The product signal during the ith hop

Using the trigonometric identity

A bandpass filter is used to block the difference frequency and pass the sum frequency, yielding an FHSS signal of

Frequency Hoping Spread Spectrum At a receiver

A bandpass filter is used to block the sum frequency and pass the difference frequency

The same form as

Multiple Frequency-Shift Keying (MFSK)

More than two frequencies are used More bandwidth efficient but more susceptible to error

f i = f c + (2i – 1 – M)f d

f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L

L = number of bits per signal element

tfAts ii 2cos Mi 1

FHSS Using MFSK MFSK signal is translated to a new frequency

every Tc seconds by modulating the MFSK signal with the FHSS carrier signal

For data rate of R: duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds

Tc Ts - slow-frequency-hop spread spectrum multiple data bits per frequency hop

Tc < Ts - fast-frequency-hop spread spectrum multiple frequency hops per data bit

Slow Frequency Hop Spread Spectrum Using MFSK (M = 4, k = 2)

Multiple Frequency-Shift Keying (MFSK)

total MFSK bandwidth Wd = Mfd

Total FHSS bandwidth Ws = 2k Wd

Fast Frequency Hop Spread Spectrum Using MFSK (M = 4, k = 2)

FHSS Performance Considerations Large number of frequencies used Large value of k results in a system that is quite resistant to

jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency

band Ex. MFSK transmitter with BW Wd and noise jammer of the same

BW then Eb/Nj = EbWd/Sj

If frequency hopping is used, jammer must jam 2k frequencies. With fixed power, jammed frequency will be reduced Sj/2k.

The gain in SNR or processing gain is Gp = 2k = Ws/Wd

Direct Sequence Spread Spectrum (DSSS)

Each bit in original signal is represented by multiple bits in the transmitted signal

Spreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits used

One technique combines digital information stream with the spreading code bit stream using exclusive-OR (Figure 7.6)

Direct Sequence Spread Spectrum (DSSS)

Phase-Shift Keying (PSK) Two-level PSK (BPSK)

Uses two phases to represent binary digits

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos

tfA c2cos1binary 0binary

Phase-Shift Keying (PSK) BPSK signal can be represented

DSSS Using BPSK Multiply BPSK signal,

sd(t) = A d(t) cos(2 fct)

by c(t) [takes values +1, -1] to gets(t) = A d(t)c(t) cos(2 fct)

At receiver, incoming signal multiplied by c(t) Since, c(t) x c(t) = 1, incoming signal is recovered

DSSS Using BPSK

DSSS Using BPSK

DSSS Performance Consideration

DSSS Performance Consideration

assume a simple jamming signal at the center frequency of the DSSS system with a form

the received signal is

where

DSSS Performance Consideration

The despreader at the receiver multiplies sr(t) by c(t), for jamming

Which is simply a BPSK modulation of the carrier tone

the carrier power Sj is spread over a bandwidth of approximately 2/Tc

Using Bandpass filter with BW 2/T, most of the jamming power is filtered

DSSS Performance Consideration

as an approximation, we can say that the jamming power passed by the filter is

The jamming is reduced by (Tc/T) The inverse of this factor is the gain in SNR

Code-Division Multiple Access (CDMA)

CDMA is a multiplexing technique used with spread spectrum

Basic Principles of CDMA D = rate of data signal Break each bit into k chips

Chips are a user-specific fixed pattern Chip data rate of new channel = kD

CDMA Example

CDMA Example If k=6 and code is a sequence of 1s and -1s

For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6>

For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6>

Receiver knows sender’s code and performs electronic decode function

u is the user that we are interested in <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code

665544332211 cdcdcdcdcdcddSu

CDMA Example User A code = <1, –1, –1, 1, –1, 1>

To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1>

User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1>

Receiver receiving with A’s code (A’s code) x (received chip pattern)

User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored

CDMA Example the preceding computation using SA

becomes

If A sends a 0 bit that corresponds to

no matter what sequence of 1s and -1s, it is always

CDMA Example If B sends a 1 bit, then

Thus, the unwanted signal (from B) does not show up at all

if the decoder is linear and if A and B transmit signals SA and SB , respectively, at the same time, then

The codes of A and B that have the property that

are called orthogonal

In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise

CDMA Example User’s codes

Transmission from A

CDMA Example Transmission from B, receiver attempts to recover

A’s transmission

Transmission from C, receiver attempts to recover B’s transmission

CDMA Example Transmission from B and C, receiver

attempts to recover B’s transmission

CDMA for Direct Sequence Spread Spectrum

Categories of Spreading Sequences

Spreading Sequence Categories PN sequences Orthogonal codes

For FHSS systems PN sequences most common

For DSSS systems not employing CDMA PN sequences most common

For DSSS CDMA systems PN sequences Orthogonal codes

PN Sequences PN generator produces periodic sequence that

appears to be random PN Sequences

Generated by an algorithm using initial seed Sequence isn’t statistically random but will pass

many test of randomness Sequences referred to as pseudorandom numbers

or pseudonoise sequences Unless algorithm and seed are known, the

sequence is impractical to predict

Important PN Properties Randomness

Uniform distribution Balance property :

in long sequence the fraction of binary ones should approach 1/2 Run property: run is defined as a sequence of all 1-s or a sequence

of all 0-s about 1/2 of runs of each type should be of length 1,1/4 of length 2,

1/8 of length 3, etc. Independence

no one value in sequence can be inferredfrom the others Correlation property

good auto-and cross-correlation properties

Unpredictability

Implementation of PN sequences

Linear Feedback Shift Register Implementation

Problem Assignments

Solve all the review questions Try the following problems: 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7